The present invention relates to a discharge lamp lighting system, and more particularly to a circuit arrangement suitable for the system which uses a power supply with a large voltage variation.
A conventional discharge lamp lighting circuit has the fundamental arrangement as shown in FIG. 2 of Japanese Utility Model Laid-open Publication JP-U-61-57500. This circuit is generally called a push-pull inverter wherein a pair of switching devices are alternately turned on and off at the frequency determined mainly by the values of the inductance of a transformer and a resonant capacitor to convert a d.c. voltage into a high frequency a.c. voltage which is boosted by the transformer for application to the discharge lamp and for start of lighting the discharge lamp. Thereafter, the discharge lamp is maintained turned on with the help of a ballast.
The above conventional technique does not pay attention to a power supply of the type having a large voltage variation, such as a power supply obtained through rectification and smoothing of an output from a dynamo mounted on a bicycle. In particular, when the power supply voltage lowers, a voltage high enough to cause the discharge lamp to start discharge cannot be supplied, thus posing a problem of short life of the discharge lamp because a high voltage continues to be applied across the discharge lamp.